In order to improve the rolling accuracy, it is necessary to know with a high accuracy the mill modulus of a mill under rolling process and the offset value of the roll gap of a rolling mill. However, since the mill modulus, the offset value and the reaction force at the start and during rolling are different, it is necessary to update those values with the most reliable and latest values in accordance with the actual rolling state, which is discussed in the paper entitled "Mill modulus variation and hysteresis - Their effect on hot strip mill AGC" by G. E. Wood published in Iron and Steel Engineer, January 1977, pages 65-70. Further in the paper entitled "Adaptive control" by Torsten Cegrell, ASEA JOURNAL, 1978, Vol. 51, No. 3, pages 75-77: says "it is possible to determine the parameter values with the aid of the technique of recursive least squares," and shows therein theoretical grounds of this statement.
Referring to FIG. 1, there is shown a sectional view of a rolling mill in an ordinary rolling process, in which a top work roll (TR) 1 and a bottom work roll (BR) 2 of the rolling mill are opposed to each other with a gap S forming therebetween. A rolling material (m) 3 is rolled to a plate of thickness h as it moves to the right in the figure. At this time, the rolls receive a reaction force F from the material to force the gap S to be changed. The relationship between the roll gap S and the reaction force F is shown in FIG. 2, in which the roll gap S increases as the reaction force increases, as indicated by line A and in which the gradient M of the line represents a mill modulus which corresponds to the magnitude of rigidity of the rolling mill. The plate thickness h has the following relationship with the roll gap S and the reaction force F: EQU h=F/m+S+K (1)
wherein K represents an offset value of the roll gap.
In conventional rolling mills, however, the mill modulus M and the offset value K are fixedly given, even if those values may change in the actual rolling state so that a high rolling accuracy has heretofore been unobtainable.